fix: fix install requires python_version constraint for protobuf (backport #3805) (#3821)

This is an automatic backport of pull request #3805 done by [Mergify](https://mergify.com).
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	both modified:   setup.py

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Author: mergify[bot]

setup.py

@@ -241,7 +241,7 @@ def get_exts_for(name):
         "typing; python_version<'3.5'",
         "packaging>=17.1",
         "protobuf>=3; python_version>='3.7'",
-        "protobuf>=3,<4.0; python_version<'3.7'",
+        "protobuf>=3,<4.0; python_version=='3.6'",
         "protobuf>=3,<3.18; python_version<'3.6'",
         "tenacity>=5",
         "attrs>=19.2.0",